1. Field of the Invention
The present invention relates generally to aircraft, and more specifically to rotorcraft. Even more specifically, the present invention relates to rotorcraft with plural lifting rotors.
2. Discussion of the Related Art
A rotorcraft is a heavier-than-air flying machine that uses life generated by rotor blades that revolve around a mast, e.g., a helicopter. A multirotor aircraft is a rotorcraft with more than one rotors. Multirotor aircraft have advantages over traditional airplanes and helicopters, such as fewer landing requirements, less cost, and less complexity.
Conventional multirotor aircraft with a low number of rotors are inherently unsafe for carrying a person due to their complexity and lack of redundancy. The worst-case count is the quad-copter (with four rotors) due to its geometric inability to balance thrust in a stable configuration when performing an emergency landing. Rotor size in multirotor vehicles is limited due to the nature of the multirotor fixed-pitch/variable rotation rate design approach. To maintain vehicle stability in a multirotor aircraft, a flight-controller needs to vary rotor RPM rapidly, and rotor inertia becomes a key limiting factor. As rotor diameter increases, drive requirements increase more rapidly and become sub-optimal in efficiency (power and weight) terms, thus limiting maximum feasible size when using a conventional frame design. Larger rotors contain more kinetic energy, making them more dangerous and also encounter speed-of-sound limitations as diameter and RPM increase.
Multirotor flying vehicles typically include a series of engineering and design tradeoffs that enable them to operate within the parameters required for them to fly. For example, the fixed-pitch rotor is a conventional characteristic of multirotor aircraft. Instead of altering a variable rotor pitch to control thrust as a conventional one-rotor helicopter, multirotor vehicles use three or more simple fixed-pitch rotors. An electronic controller rapidly alters the rotors' rotational velocity to alter the rotors' thrust, altering the aggregate center of thrust as opposed to the vehicle's center of gravity (which typically does not move while in flight), maintaining stability in flight. This is a simpler overall design mechanical design compared to a variable-pitch vehicle. However, maximum rotor size is limited in a fixed-pitch/variable-speed aircraft due to the conflict between minimum thrust-adjustment (rotor speed) reaction time requirements to maintain vehicle stability vs. motor size/power supply/efficiency limitations.
The typical hub-and-spoke (H&S) frame layout does not scale up rotor count beyond a certain level due to limitations built in to the design approach. Typical multirotor aircraft designs specify a central hub containing electronics, etc. and have a number of arms extending outward from the center to position rotors, usually roughly forming a circular pattern around the hub. The H&S layout may be optimal for low-count (3-8) rotors, but beyond a threshold diminishing returns increase rapidly due to several factors. Primarily this because if the vehicle designed for lift in the +Z (vertical) axis, rotors arranged on the XY plane around the Z axis are necessary, with each rotor requiring clearance (tip to tip) on the XY axis so rotor capacity is a function of rotor diameter and distance from the hub. Support structure (frame) strength requirements increase with hub distance, and although attempts to optimize spoke structure may somewhat mitigate the inherent limitations of the hub-and-spoke arrangement, scaling upwards becomes impractical.
The combination of limitations fixed-pitch rotors and H&S frames severely limits overall vehicle capacity and capabilities. Given that a maximum rotor size exists given current state of the art, the best avenue for improving capacity and performance is in re-examining the approach to frame design.